In the continuing research for better methods of machining metals with lower costs, particularly by use of ceramics, there has been some lack of understanding of the interdependence of thermal, physical and mechanical properties of materials needed to machine a specific metal.
Ceramic materials used to date as tool bits for cutting by the prior art have fallen into two classes: (a) Al.sub.2 O.sub.3 based ceramics that contain 10-20% of a densifying agent such as TiN, TiO.sub.2, CaO, MgO or Cr.sub.2 O.sub.3, and (b) Si.sub.3 N.sub.4 based ceramics that contain a minor amount (2-20%) of a densifying agent such as MgO or Y.sub.2 O.sub.3. Almost all of today's available commercial ceramic cutting tools are of the class (a) type. Al.sub.2 O.sub.3 based commercial cutting tools vary widely in their tool life and fail by thermal cracking or chipping attributed to inherent brittleness. The quick cure by the prior art to extend tool life has been to concentrate on physical or mechanical properties by making more rigid the holding of the tool while machining, eliminating vibrations in the machining setup, and by improving tool geometry. The poor thermal shock resistance of Al.sub.2 O.sub.3 based ceramics was ignored. Despite a variable tool life, Al.sub.2 O.sub.3 based ceramics have shown excellent chemical stability and thereby superior resistance to cratering resulting from chemical reactivity with the hot chip being removed.
Certain Si.sub.3 N.sub.4 based ceramics possess sufficient thermal shock resistance to dramatically improve tool life when machining metals that do not present a chemical reactivity problem, such as grey cast iron. However, when used to machine steel or nodular iron, the Si.sub.3 N.sub.4 based material exhibit poor resistance to wear or surface abrasion at elevated temperatures such as 1000.degree.-1200.degree. C. and exhibit poor resistance to oxidation. The tool material wears out readily as evidenced by severe crater wear. Similar cratering or wear patterns are experienced on machining steel or nodular iron, regardless of the densifying additives employed. This suggests that such poor wear resistance when cutting steel or nodular iron is an inherent defect of Si.sub.3 N.sub.4. It is believed that the existence of an extremely hot stringer or chip of stock metal, which is unsevered but shear cut by the tool, comes back into contact with the tool bit surface creating the cratering wear. In machining grey cast iron, there is very little extended chip formation that can come back or remain in contact with the tool so the circumstance for promoting cratering is avoided.
What is needed is an improved method of machining steel, nodular cast iron, or malleable cast iron, which metals each present a chemical reactivity problem. The method should utilize a ceramic material that is chemically stable, has a high thermal shock factor, and is reasonably strong at elevated tool bit temperatures.